Compositions for the removal of silicone conformal coatings from a printed circuit board

ABSTRACT

A solvent composition has an oxygenated solvent and a siloxane solvent. In one embodiment, the oxygenated solvent is propylene glycol methyl ether and the siloxane solvent is hexamethyldisiloxane or octamethyltrisiloxane. In another embodiment, the solvent composition is an azeotrope of propylene glycol n-butyl ether and decamethyltetrasiloxane. The siloxane solvent can be used in any situation where one desires to remove a silicone deposit, e.g., conformal coatings, adhesives, sealants, greases, heat transfer fluids, paints, oils, etc.

CLAIM OF DOMESTIC PRIORITY

The present application is a continuation of U.S. patent applicationSer. No. 16/704,208, filed Dec. 5, 2019, which claims the benefit ofU.S. Provisional Application No. 62/852,950, filed May 24, 2019, whichapplication are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to compositions for the removal of curedor uncured silicone deposits, methods of making, and methods of usingthe compositions.

BACKGROUND OF THE INVENTION

Silicone is commonly used in a wide variety of fields. Silicone is usedto provide electrical insulation, as a protective coating overelectronics, as a sealant in household, automotive, or aerospace uses,and in innumerable other situations. Silicone comes into play in almostevery conceivable industry.

Silicone-containing fluids can be reactive or non-reactive dependingupon the formulation and desired end use. These fluids are selected foruse due to their versatility, durability, and water and chemicalresistance. Forms associated with reactive and non-reactive siliconefluids range from uncured oils, greases, and heat transfer fluids, tocured sealants, adhesives, paints, and conformal coatings. Allsilicone-containing fluids are notoriously difficult to clean or removefrom industrial and surface mount technology surfaces without damagingthe substrates, or without requiring high temperatures and specializedequipment.

Uncured silicone fluids typically require cleaning solution temperaturesabove the silicone fluid softening point, which is often unattainable intypical cleaning applications. Cured silicone fluids typically require adigestant in aqueous media to break the polymeric silanol network, andthese common digestants are potentially damaging to the substrate towhich they are attached since silanol bonds of cured silicone fluids areformed with the active substrate as well.

Siloxane solvents have been used to remove silicone deposits, becausesiloxane reacts with silicone. However, using siloxane solvents involvesat least two major drawbacks. First, siloxane solvents are prohibitivelyexpensive for most use-cases. Second, a lot of silicone adhesives,coatings, greases, etc., include other ingredients besides purelysilicone. The additives reduce the effectiveness of the siloxanesolvents.

Therefore, a need exists for improved solvent compositions for removalof silicone deposits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b illustrate mixing a plurality of solvents to form anew solvent composition; and

FIGS. 2 a-2 h illustrate removing a conformal silicone coating from aPCB using the new solvent composition.

DETAILED DESCRIPTION

The present invention is described in one or more embodiments in thefollowing description with reference to the figures, in which likenumerals represent the same or similar elements. While the invention isdescribed in terms of the best mode for achieving the invention'sobjectives, it will be appreciated by those skilled in the art that itis intended to cover alternatives, modifications, and equivalents as maybe included within the spirit and scope of the invention as defined bythe appended claims and their equivalents as supported by the followingdisclosure and drawings.

Combining a volatile siloxane solvent with an oxygenated solvent in thediol or glycol ether class creates a broad-spectrum silicone depositremoval solution that acts on a wide variety of both cured and uncuredsilicone fluids or deposits, does not require extreme temperatures toreach the silicone fluid softening points, and can interrupt thepolymeric silanol network of cured silicone fluids without damagingsubstrate material.

The oxygenated solvent is selected from the following non-exhaustivelist in some embodiments: ethylene glycol, diethylene glycol,triethylene glycol, propylene glycol, dipropylene glycol, tripropyleneglycol, butylene glycol, dibutylene glycol, tributylene glycol,pentylene glycol, dipentylene glycol, tripentylene glycol, hexyleneglycol, dihexylene glycol, trihexylene glycol, heptylene glycol,diheptylene glycol, triheptylene glycol, octylene glycol, dioctyleneglycol, trioctylene glycol, propylene glycol methyl ether, dipropyleneglycol methyl ether, tripropylene glycol methyl ether, propylene glycolmethyl ether acetate, dipropylene glycol methyl ether acetate, propyleneglycol n-propyl ether, dipropylene glycol n-propyl ether, propyleneglycol n-butyl ether, dipropylene glycol n-butyl ether, tripropyleneglycol n-butyl ether, propylene glycol phenyl ether, propylene glycoldiacetate, dipropylene glycol dimethyl ether, diethylene glycol ethylether, diethylene glycol methyl ether, diethylene glycol n-butyl ether,diethylene glycol hexyl ether, diethylene glycol n-butyl ether acetate,ethylene glycol propyl ether, ethylene glycol n-butyl ether, ethyleneglycol hexyl ether, ethylene glycol n-butyl ether acetate, triethyleneglycol methyl ether, triethylene glycol ethyl ether, triethylene glycoln-butyl ether, ethylene glycol phenyl ether, and ethylene glycol n-butylether.

The siloxane solvent is selected from the following non-exhaustive listin some embodiments: hexamethyldisiloxane, octamethyltrisiloxane,decamethyltetrasiloxane, dodecamethylpentasiloxane,polydimethylsiloxane, phenylmethylsiloxane, dimethyl-diphenylsiloxane,cyclomethicone, octamethylcyclotetrasiloxane,decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, andpolymethylhydrogensiloxane.

To create the new solvent composition, any of the desired solvents aremixed together, with at least one oxygenated solvent and one siloxanesolvent in the mixture. FIGS. 1 a and 1 b illustrates mixing twosolvents two create a new solvent composition 100. An oxygenated solvent110 and a siloxane solvent 112 are each poured into a beaker or othercontainer 120 in FIG. 1 a . Any suitable container for mixing solventscan be used instead of beaker 120, e.g., a vat, a bucket, a bottle, abarrel or drum, etc. Oxygenated solvent 110 and siloxane solvent 112 canbe poured from beakers, bottles, or another suitable container asillustrated, from a machine with a fluid conduit and nozzle, or via anyother suitable fluid dispensing mechanism. Oxygenated solvent 110 andsiloxane solvent 112 can be dispensed simultaneously or one after theother with either solvent added to the other.

Oxygenated solvent 110 and siloxane solvent 112 will typically be ableto thoroughly mix simply by adding the two solvents into a containertogether. However, in some cases the solvents may need to be agitated tofully mix into solvent composition 100. FIG. 1B illustrates agitatingusing a mixing rod 130. Mixing rod 130 is inserted into beaker 120 andmoved around to cause oxygenated solvent 110 and siloxane solvent 112 tomix together into solvent composition 100. The solvents can be agitatedby inserting any suitable mixing device into beaker 120, e.g., a spoon,spatula, or paint mixer. In some embodiments, a motor is attached to themixing device to automatically agitate the solvents. The solvents canalso be mixed by agitating beaker 120 while both solvents are containedin the beaker, e.g., by placing the beaker on a moving platform or bypicking up and moving the beaker around.

Solvent composition 100 is shown as a binary solvent, with twoconstituent solvents. However, solvent composition 100 can also beformed as a ternary solvent with three constituent solvents, aquaternary solvent with four constituent solvents, or even more solventscould be combined. The key is that at least one oxygenated solvent andone siloxane solvent is used. Binary solvents operate better in somesituations than mixing a higher number of solvents because the smallersolvent molecules in a binary solvent will get into the microscopicpores of the silicone material easier. Steric hinderance can become aproblem with some ternary or quaternary solvents for some uses. However,having the additional qualities of more solvents in solvent composition100 may be preferable in other use-cases.

One consideration for selecting the specific solvents to combine isboiling point, evaporation rate, or vapor pressure. For most use-cases,a faster evaporating solvent is desirable so that the solvent evaporatesoff of the substrate quicker. Therefore, constituent solvents with afaster evaporation rate, lower vapor pressure, or lower boiling pointcan be selected to reduce the amount of time the solvent composition 100remains on the substrate after application.

Of the above-listed solvents, propylene glycol methyl ether is thefastest evaporating oxygenated solvent, and hexamethyldisiloxane is thefastest evaporating siloxane solvent. Therefore, a mixture withpropylene glycol methyl ether as oxygenated solvent 110 andhexamethyldisiloxane as siloxane solvent 112 makes a suitable solventcomposition 100 with a relatively fast evaporation rate.Octamethyltrisiloxane is the second fastest evaporating siloxane solventof the above-listed solvents. A mixture of propylene glycol methyl etheras oxygenated solvent 110 and octamethyltrisiloxane as siloxane solvent112 would also make a suitable solvent composition 100 with a relativelyfast evaporation rate.

Oxygenated solvent 110 and siloxane solvent 112 can be mixed in anyratio. Siloxane solvent 112 is generally more expensive than oxygenatedsolvent 110. Therefore, a lower percentage of siloxane solvent 112 willgenerally reduce the overall cost of solvent composition 100. However,lowering the percentage of siloxane solvent 112 below a lower thresholdwill undesirably reduce the effectiveness of solvent composition 100 atdissolving silicone. Reducing the percentage of siloxane solvent 112below about 15% by weight has been found to affect the ability ofsolvent composition 100 to dissolve silicone, while keeping thepercentage at or above 15% has not had a significant impact on thecapabilities of the solvent composition. Therefore, in one embodiment,solvent composition 100 is formed with 15% by weight or greater ofsiloxane solvent 112.

In some cases, a particular oxygenated solvent 110 matched with aparticular siloxane solvent 112 forms an azeotrope. An azeotrope is amixture of two or more liquids whose proportions cannot be altered orchanged by simple distillation. This happens because when an azeotropeis boiled, the resultant vapor has the same proportions of constituentsas the unboiled mixture. To form an azeotrope, a certain percentage ofeach constituent solvent is required depending on the particularsolvents selected. For some combinations of particular oxygenatedsolvents 110 and siloxane solvents 112, an azeotrope is not possible.For other solvents, the azeotrope may have undesirable properties, suchas a slower evaporation rate than the constituents or requiring apercentage of siloxane solvent 112 that is too low for the solventcomposition 100 to be effective.

One particular effective azeotrope is a mixture using propylene glycoln-butyl ether for oxygenated solvent 110 and decamethyltetrasiloxane forsiloxane solvent 112. An azeotrope is formed when mixed in the ratio of15% decamethyltetrasiloxane to 85% propylene glycol n-butyl ether byweight. While neither of the individual solvents in the azeotrope arethe fastest evaporating from their respective lists above, the azeotropehas a faster evaporating rate than either of the constituent solvents.The azeotrope formed with 15% decamethyltetrasiloxane and 85% propyleneglycol n-butyl ether has an evaporation rate that is on par with anyother known combination.

Once solvent composition 100 has been mixed using any of the abovedescribed combinations, the solvent composition can be used in anysituation where one desires a silicone deposit be removed. FIGS. 2 a-2 hillustrate one situation in which solvent composition 100 isparticularly useful. FIG. 2 a shows an exemplary printed circuit board(PCB) 200. PCB 200 can have any combination of electrical componentsmounted onto the PCB. PCB 200 can be a stand-alone electrical systemthat uses the semiconductor packages to perform one or more electricalfunctions, such as an HVAC controller or a vehicle electronic controlunit. Alternatively, PCB 200 can be a subcomponent of a larger system.For example, PCB 200 can be part of a tablet computer, cellular phone,digital camera, communication system, or other electronic device. PCB200 can also be a graphics card, network interface card, or anothersignal processing card that is inserted into a computer.

In FIG. 2 a , a variety of semiconductor packages 210 and surface mountcomponents 212 are mounted onto PCB 200 to form an electrical system. Ingeneral, any suitable component can be used, such as semiconductorpackages with integrated circuits or discrete active or passive partssuch as diodes, transistors, inductors, or capacitors. Semiconductorpackages 210 can include microprocessors, memories, ASICs, logiccircuits, analog circuits, RF circuits, or other semiconductor die orelectrical components. Any other desired electrical component can beused as needed for a desired electrical functionality to be performed byPCB 200.

In FIG. 2 a , PCB 200 has been completely assembled and is functionallycomplete. FIG. 2 b shows a conformal silicone coating 220 being appliedby a spray applicator with nozzle 222. Nozzle 222 releases a mist 224 ofsilicone material that forms conformal silicone coating 220. Applyingconformal silicone coating 220 is a common finishing step to protect acompleted PCB from water and other contaminates when deployed.

FIG. 2 c shows a cross-section of PCB 200 with one of the surface mountcomponents 212 covered in silicone coating 220. One reason that siliconecoating 220 may be removed is to replace or upgrade one of the parts.For example, component 212 might be a resistor that has blown out andcreated an open circuit. A semiconductor package 210 may be replaced toupgrade microcontroller capabilities or memory size. Rather thancompletely replace PCB 200, components 210 and 212 can be replaced torepair or upgrade the PCB. However, replacing a component is difficultwithout first removing coating 220 over the component.

Solvent composition 100 allows removal of coating 220. First, solventcomposition 100 is applied onto coating 220. FIG. 2 d shows one simplemethod of applying solvent composition 100 to a small localized areausing dropper 230. With dropper 230, coating 220 can be removed over asingle component 210 or 212, or a small grouping of components, withoutremoving the coating over the rest of PCB 200.

Solvent composition 100 is deposited onto coating 220 over component 212in FIG. 2 d and immediately begins to dissolve the coating. The siloxanesolvent 112 in the mixture attacks the silicone. Oxygenated solvent 110not only reduces the cost of solvent composition 100, but also improvesthe dissolving action. When coating 220 is applied, there are usuallycuring agents mixed in with the silicone. The curing agents absorb waterto harden coating 220, which forms a silanol network within the coating.The silanol network provides a relatively high attraction of coating 220to PCB 200 but is not dissolved by siloxane solvent 112. Oxygenatedsolvent 110 in solvent composition 100 destroys the silanol network incoating 220, resulting in a much faster dissolving of the coating thanhaving only siloxane solvent 112 to dissolve the silicone portion.Oxygenated solvent 110 also helps remove other impurities such as waterand other polarized molecules or flux left over from the manufactureprocess. Solvent composition 100 can also be used to remove flux duringmanufacture of electronic devices prior to deposition of coating 220.

Solvent composition 100 can be applied using any other suitable method.FIGS. 2 e and 2 f illustrate two other non-limiting methods forapplication. In FIG. 2 e , solvent composition 100 is disposed in anaerosol spray can 232 and sprayed onto PCB 200. Aerosol can 232 allowsapplication of solvent composition 100 over a larger area of PCB 200than with dropper 230 while still allowing localized application. A useraims the spray stream from aerosol can 232 at a particular component orgroup of components to remove coating 220 over those componentspecifically. Alternatively, aerosol can 232 can be used to applysolvent composition 100 over an entire surface of PCB 200.

In one embodiment, aerosol can 232 is used to apply solvent composition100 over an area of PCB 200, and then the solvent composition is allowedto set on top of coating 220 to dissolve the coating. In anotherembodiment, aerosol can 232 is used to apply a constant spray of solventcomposition 100 onto an area of PCB 200. The continuing spray of solventcomposition 100 actively dissolves and washes away coating 220 while thestream flows over PCB 200. Aerosol can 232 can be held at a low anglerelative to PCB 200 to spray solvent composition 100 under componentsand in other tight areas on the PCB, which helps to fully remove coating220.

For some configurations of solvent composition 100, the ratio ofsiloxane solvent 112 to oxygenated solvent 110 will need to be increasedto provide an adequate stream of the solvent composition from theaerosol nozzle. For a typical aerosol can 232, the mixture of solventcomposition 100 may need to be at least 50% by weight of siloxanesolvent 112 to create an adequate stream of the solvent composition. Thestream could also potentially be improved by modifying aerosol can 232or by picking different constituent solvents.

In FIG. 2 f , PCB 200 is disposed into a bath 234 of solvent composition100. Bath 234 is a container filled with solvent composition 100sufficiently to fully cover PCB 200. PCB 200 soaks in bath 234 whilesolvent composition 100 dissolves coating 220. In some embodiments, apump is used to create a flow of solvent composition 100 over PCB 200.

In FIG. 2 g , a brush or other tool 236 is used to supplement thedissolving action of solvent composition 100. Solvent composition 100 isfirst applied using any suitable application method, including thosediscussed above. After waiting a sufficient time for coating 220 todissolve, typically a few seconds or minutes, brush 236 can be used toscrub off coating 220. The bristles of brush 236 help remove partiallydissolved portions of coating 220 and distribute solvent composition 100to nooks and crannies where a thicker buildup of the coating wasdeposited. In other embodiments, brush 236 is used to apply solventcomposition 100 without first relying on another application method bydipping the brush in 236 in solvent composition 100 or otherwiseapplying the solvent composition to the brush.

FIG. 2 h shows component 212 with coating 220 removed over thecomponent. Component 212 can easily be desoldered and replaced with aworking component to repair PCB 200. After component 212 is replaced,coating 220 can be reapplied over the component to make sure that PCB200 is fully protected. Coating 220 can be applied by spraying asilicone fluid over the entire PCB 200 as shown in FIG. 2 b , or justthe area where the coating was removed. Coating 220 can also be replacedby brushing an uncured silicone fluid over component 212, or by anyother suitable method.

Using solvent composition 100, with a mixture of oxygenated solvent 110and siloxane solvent 112, to remove coating 220 reduces cost andproblems of PCB rework. Coating 220 is dissolved faster so solventcomposition 100 can be removed quicker than with purely siloxane solvent112. Having solvent on PCB 200 for less total time reduces thelikelihood of damage to the PCB. The addition of oxygenated solvent 110not only removes coating 220 faster by attacking the silanol networkwithin the coating, but also helps remove other contaminants thatsiloxane solvent 112 alone would not remove.

Solvent composition 100 is usable and provides benefits in any situationwhere a silicone substance needs to be removed. Misting applicatormachines, such as the one shown in FIG. 2 b with nozzle 222, commonlyneed to be cleaned. For instance, if a different liquid besides theuncured silicone fluid needs to be sprayed then the existing siliconewithin and on the machine will need to be fully removed. Solventcomposition 100 can be used to clean nozzles, flow lines, holding tanks,and other parts of misting machines. Misting can also create a mess onthe outside of the machine, or on nearby objects, that can be cleanedwith solvent composition 100.

Silicone calking is commonly used in a variety of industries and aroundthe home as a sealant. Silicone calking, which is notoriously difficultto remove cleanly, can be removed with solvent composition 100. Siliconecalking may need to be removed when replacing one of the components thatthe silicone calking contacts, or just to replace the silicone calkingif the seal fails. Solvent composition 100 can also be used to removesilicone-based adhesives, greases, etc. Siloxane solvent 112 breaks downthe silicone while oxygenated solvent 110 dissolves other additivescommonly used.

Silicone heat transfer fluids are commonly used in a wide variety ofindustries but are problematic to remove or clean. Silicone heattransfer fluid can require cleaning because of an inadvertent spill thatcreates a mess. In other cases, a silicone heat transfer fluid isintentionally applied to parts for testing purposes, and then needs tobe cleaned.

Solvent composition 100 is also useful in uranium extraction. Siliconefluids are used to coat beads of material that uranium is to beextracted from. The silicone fluids expand the beads and pick up uranylnitride that is created. Using solvent composition 100 removes thesilicone from the beads effectively and economically.

While one or more embodiments of the present invention have beenillustrated in detail, the skilled artisan will appreciate thatmodifications and adaptations to those embodiments may be made withoutdeparting from the scope of the present invention as set forth in thefollowing claims.

What is claimed:
 1. A method of removing a silicone conformal coatingfrom a printed circuit board, comprising: A) providing a solventcomposition including an oxygenated solvent and a siloxane solvent; B)providing a printed circuit board (PCB) including a first electricalcomponent mounted to the PCB and a silicone conformal coating depositedover the PCB and first electrical component; C) removing only a portionof the silicone conformal coating over the first electrical component byusing the solvent composition to dissolve the portion of the siliconeconformal coating; D) replacing the first electrical component with asecond electrical component after removing the portion of the siliconeconformal coating; and E) replacing the portion of the siliconeconformal coating with a second silicone conformal coating.
 2. Themethod of claim 1, further including applying the solvent composition asan aerosol stream.
 3. The method of claim 1, wherein the siloxanesolvent includes hexamethyldisiloxane or octamethyltrisiloxane.
 4. Themethod of claim 1, wherein the solvent composition includes at least 15%of the siloxane solvent by weight.
 5. The method of claim 1, wherein theoxygenated solvent includes propylene glycol n-butyl ether and thesiloxane solvent includes decamethyltetrasiloxane.
 6. The method ofclaim 1, wherein the solvent composition is an azeotrope.
 7. The methodof claim 1, further including selecting the siloxane solvent from agroup consisting of hexamethyldisiloxane, octamethyltrisiloxane,decamethyltetrasiloxane, dodecamethylpentasiloxane,polydimethylsiloxane, phenylmethylsiloxane, dimethyl-diphenylsiloxane,cyclomethicone, octamethylcyclotetrasiloxane,decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane, andpolymethylhydrogensiloxane.
 8. The method of claim 7, further includingselecting the oxygenated solvent from a group consisting of ethyleneglycol, diethylene glycol, triethylene glycol, propylene glycol,dipropylene glycol, tripropylene glycol, butylene glycol, dibutyleneglycol, tributylene glycol, pentylene glycol, dipentylene glycol,tripentylene glycol, hexylene glycol, dihexylene glycol, trihexyleneglycol, heptylene glycol, diheptylene glycol, triheptylene glycol,octylene glycol, dioctylene glycol, and trioctylene glycol.
 9. Themethod of claim 7, further including selecting the oxygenated solventfrom a group consisting of propylene glycol methyl ether, dipropyleneglycol methyl ether, tripropylene glycol methyl ether, propylene glycolmethyl ether acetate, dipropylene glycol methyl ether acetate, propyleneglycol n-propyl ether, dipropylene glycol n-propyl ether, propyleneglycol n-butyl ether, dipropylene glycol n-butyl ether, tripropyleneglycol n-butyl ether, propylene glycol phenyl ether, propylene glycoldiacetate, dipropylene glycol dimethyl ether, diethylene glycol ethylether, diethylene glycol methyl ether, diethylene glycol n-butyl ether,diethylene glycol hexyl ether, diethylene glycol n-butyl ether acetate,ethylene glycol propyl ether, ethylene glycol n-butyl ether, ethyleneglycol hexyl ether, ethylene glycol n-butyl ether acetate, triethyleneglycol methyl ether, triethylene glycol ethyl ether, triethylene glycoln-butyl ether, ethylene glycol phenyl ether, and ethylene glycol n-butylether.
 10. The method of claim 1, further including applying the solventcomposition using a dropper.
 11. The method of claim 1, furtherincluding scrubbing the portion of the silicone coating with a brushwhile the solvent composition is disposed on the portion of the siliconecoating.